1. Field of the Invention
The present invention relates to a method of manufacturing an optical information recording medium with high density, for example, with a light-transmitting layer thereof made to be thin.
2. Description of the Prior Art
Recently, research relating to various types of optical information recording has been studied in the optical information recording field. Optical information recording methods have been emerging which are applicable to a range of uses, because these methods can allow media to have higher density, can record/reproduce information by a non-contact method, and can also achieve these objectives at a low price. Currently, an optical disc has a structure produced, for example, by forming an information layer on a transparent resin layer with a thickness of 1.2 mm, and then covering and protecting the layer with an over coating (which is compact disc (CD)), or by forming information layers on one or both sides of a transparent resin layer with an over coating, or by forming information layers on one or both sides of a transparent resin layer with a thickness of 0.6 mm, and then laminating two of the information layers (which is Digital Versatile Disc (DVD)).
Recently, as a way to increase the recording density of optical discs, methods have been studied such as increasing the numerical aperture (NA) of an objective lens, and shortening the wavelength of the laser. In these methods, if the thickness of a recording/reproducing base material (which is a substrate of the side on which an optical laser is incident) is thin, the influence of an aberration of the laser spot decreases, and allowance for a gradient angle (tilt) of the disc increases. From this, an idea was proposed to set the thickness of the recording/reproducing base material to be around 0.1 mm, NA to be around 0.85, and the wavelength of a laser to be around 400 nm (see Japanese unexamined patent publication H08-235638, for example). Here, because of effects on the focus of the recording/reproducing light and risk of spherical aberration, it is preferable that the thickness variation of the recording/reproducing base material is reduced to be within 5%. Even in an optical disc having such reduced thickness variation, in which the recording/reproducing base material has thickness is 0.1 mm, the thickness of the disc is preferably 1.2 mm like a conventional CD or DVD, because the disc should have compatibility with existing hardware.
Such a base material (which is a light-transmitting layer) on the recording/reproducing side of Blu-ray discs can not be formed by injection molding which has been conventionally used for producing optical discs, because the material is as thin as approximately 0.1 mm. Generally, when the substrate has a diameter of approximately 12 cm and its thickness of is less than 0.3 mm, using the injection molding becomes an extremely difficult task. In addition, the base material of the recording/reproducing side needs to satisfy extremely high thickness accuracy. These were reasons why the following method has been mainstream: a sheet which is produced by a method like a casting is punched out in a disc shape, and then glued together with the substrate. However, with this method, the cost of the sheet material becomes really expensive, thus the optical disc becomes expensive. Therefore, in order to form the base material of the recording/reproducing side, a method as follows has been proposed: coating a radiation curing resin by a spin coating or the like, and then curing the resin (see Japanese unexamined patent publication H 10-289489, for example).
When a spin coating method is used to form a base material of the recording/reproducing side, the base material is required to have extremely high thickness accuracy. Thus, the spin coating also requires high technology, and so the manufacturing time becomes extremely long. More specifically, the reasons are as follows. When forming a base material of the recording/reproducing side made of a radiation curing resin on a substrate, because both a reflection film and a recording film are formed on the same side of the substrate, friction of the radiation curing resin may change over the disc area. Therefore, to form a thick film such as having a thickness of 100 μm by only one coating step, spreading must be performed at the low rotation speed. In addition, because the radiation curing resin is prevented from spreading out in areas having higher friction, the resin does not spread in a circular pattern, and the thickness of the light-transmitting layer easily becomes uneven. Therefore, to obtain the desired thickness accuracy, the spreading time during the coating step inevitably lengthens.
As mentioned above, if the manufacturing time lengthens, the mass production of a medium is not feasible as long as the production line increases, and this involves enormous cost for investment in facilities. In addition, the big problem is, even if the material cost becomes low, the production cost does not.